1. Field Of The Invention
This invention relates generally to a buffer for buffering a capacitively coupled output of a sensor and, more particularly, to a low-input capacitance self-biased CMOS buffer amplifier which buffers a low amplitude capacitively coupled output of a sensor to subsequent output circuitry.
2. Discussion Of The Related Art
For the most part, sensors such as pressure transducers, infrared detectors and the like, provide an AC capacitively coupled output at a very low voltage. That is, the output of these types of sensors are capacitively coupled to subsequent processing circuitry, such as an amplifier, which typically amplifies and processes the output signal to a usable form. Because the output voltages of these sensors are so small, an amplifier which receives the output signal from the sensor may destroy the signal because of poor capacitive coupling between the sensor and the amplifier circuitry. In other words, because the output capacitance of the sensor is small, the transfer of the signal from the sensor to the amplifier circuitry may be overwhelmed by the parasitic input capacitance of the amplifier circuitry. Therefore, it has been known to use buffering circuitry to buffer the output signal before applying it to the amplifier circuitry.
A second concern of coupling the capacitive output of these types of sensors to the amplifying output circuitry is the loss of signal because of DC leakage within the components of the output circuitry. Because the output voltage of the sensors is small, DC leakage occurring in the amplifier circuitry can have a serious effect on the integrity of the signal in that DC leakage will act as a filter that limits the frequency range of the output signal. In order to overcome the effects of DC leakage, it is known to incorporate metal oxide semiconductor field effect transistors (MOSFET) as the basic amplifying components in an output buffer and associated amplifier. In this application, field effect transistors have been known to be very effective in significantly eliminating DC leakage.
The main concern when designing a buffer therefore becomes the parasitic capacitance associated with the input connection to a MOSFET associated with the buffer. Typically, the output signal from the sensor is directly applied to a gate electrode of an input MOSFET. This FET includes a total input capacitance appearing at the gate of the input FET as a sum of its gate-to-source, gate-to-drain, and gate-to-substrate capacitances. Further, since buffers of this type are self powered, a mechanism needs to be included that enables the buffer to be DC biased, but does not introduce a capacitance to the input signal. Therefore, an important concern when designing an output buffer is attempting to eliminate or substantially limit these capacitances in order to maintain the integrity of the output signal.
An article, Yun, W., et al. titled "Surface Micromachined, Digitally Forced-Balanced Accelerometer With Integrated CMOS Detection Circuitry," Tech Digest Solid State Sensors and Actuators Workshop, Hilton Head, S.C., pp. 126-131, June 1992 discloses a unity gain buffer having a low input capacitance for use with an accelerometer circuit. The input buffer of this article uses a diode clamp technique in that a diode having a zero voltage is placed at the gate electrode of an input FET. The zero voltage causes the diode to have an extremely non-linear input voltage-to-diode junction capacitance which results in distortion and signal attenuation as a result of the nonlinear capacitive divider between the sensor and the diode junction capacitor. A unity gain buffer of this type requires several matched devices having a source-to-substrate voltage of the same value. Any of the source-to-substrate voltages, device sizes and DC biasing current which are not matched can potentially cause an oscillation in the buffer device, thus providing a potential drawback.
What is needed then is a buffer for buffering the output of a capacitive coupled sensor in which the buffer includes an effective zero capacitance as seen by the capacitive output of the sensor. It is therefore an object of the present invention to provide such a buffer.